1. Technical Field
The disclosure relates to a charge pump, and more particularly, to a charge pump feedback control device and method.
2. Related Art
Recently, portable electronics products have flourished in a variety of fields, and power management issues have arisen as a consequence. Due to the limited electric power of the portable electronics products, the power management IC has become a key component for managing power consumption effectively. A power management IC transforms a voltage of the battery into the different operation voltages of the sub-circuits of the portable electronics product, such as transforming 3.3V to 1.2V, or 3.3V to 2.5V, etc. The design rules for the power management IC's voltage transformation are high efficiency, high precision, low noise, and small volume. Generally speaking, there are three kinds of voltage transformer inside the power management IC: switching regulator, linear regulator, and charge pump regulator, of which the charge pump has advantages of smaller volume and lower design cost than the switching regulator and linear regulator.
A charge pump regulator has a capacitor array core, which is a circuit designed using several switches and capacitors. By controlling the switches to be opened/closed to change the connection relationship of the capacitors, the capacitors are charged or discharged by the connection and the power is transferred as different voltages. Generally, charge pumps are separated into open loop and close loop control schemas.
Usually, a conventional charge pump adopts the open loop schema. Please refer to FIG. 1A, in which the conventional charge pump includes: charge pump unit 50 and controller 60. Please also refer to FIG. 1B, in which the charge pump unit 50 includes a first switch S1, a second switch S2, a third switch S3, a fourth switch S4, a capacitor C1, where the controller 60 generates several phase control signal for the charge pump unit 50, please refer to FIG. 1C. The controller 60 generates a first phase for the charge pump unit 50 to close the switches S1 and S2, then the input voltage Vi charges capacitors C0 and C1, please refer to FIG. 1D. Please refer to FIG. 1E, in which controller 60 generates a second phase for the charge pump unit 50 to close switches S3 and S4, then the capacitor C1 discharges the resistor RL and capacitor C0. The ratio of the output voltage Vo and the input voltage Vi is derived thus:Q1−=(Vi−Vo)*C1  F.1Q1+=Vo*C1  F.2Q1−−Q1+=I*T  F.3
      I    =          Vo      RL        ,            and      ⁢                          ⁢      T        =          1      fs        ,for F.3
                    (                  Vi          -          Vo          -          Vo                )            *      C      ⁢                          ⁢      1        =                  Vo        RL            *              1        fs              ,then, F.4 is acquired as below:
                                          V            o                                V            i                          =                              C            1                                              2              ·                              C                1                                      +                          1                                                R                  L                                ·                                  f                  s                                                                                        F        ⁢        .4            
From the derivation of F.4, the relationship of the output voltage Vo and the input voltage Vi of the charge pump unit 50 are related with a switching frequency fs and a load RL. When RL is fixed, a gain of the output voltage Vo and the input voltage Vi is related with the switching frequency fs and the capacitance of the capacitor C1 according to F.4. Due to the open loop schema of the conventional charge pump regulator, when the switching frequency fs is fixed, the gain of the output voltage Vo divide input voltage Vi changes following the impedance of the resistor RL changes. That is, Vo=gain*Vi; when the output voltage Vo changes following the gain changes, the ripple voltage variation becomes too large. For this reason, the charge pump under open loop schema is only suitable for a fixed load, the output voltage Vo changes following the load changes.
Therefore, two problems typically happen when using the conventional charge pump under an open loop schema in which the switching frequency fs is fixed.
Firstly, when the switching frequency fs is fixed, if the impedance of the load RL became small, the load current becomes large and the output voltage Vo becomes small when derived by F.4. If the situation continues without control, the output voltage becomes too small to disable the connected circuit.
Secondly, when the switching frequency fs is fixed, if the impedance of the load RL became large, the load current becomes small and the output voltage Vo becomes large when derived by F.4. If the situation continues without control, the output voltage becomes too large to damage the connected components of the circuit.
Furthermore, the close loop schema charge pump design is developed to resolve the problems of open loop schema. The close loop schema charge pump has a comparative circuit to be used as a feedback control for the switching of the charge pump. Furthermore, the close loop schema charge pump adopts a feedback circuit to compare a reference voltage to generate at least one clock signal according to an input voltage. The clock signal controls switching of the charge pump to charge/discharge at least one capacitor for boosting or bucking the output voltage. The output voltage is then stable, and includes less ripple signal.
Another conventional charge pump under close loop schema is using a voltage controlled oscillator (VCO) or current controlled oscillator (CCO) to achieve close loop control. Through detecting the variation of the load voltage or current of the output end according to a detection and control circuit, and generating at least one clock signal by VCO or CCO to control switching of the charge pump to charge/discharge at least one capacitor for boosting or bucking the output voltage, the output voltage is stable and includes less ripple signal.
However, the conventional close loop schema charge pump also has several problems.
Firstly, if the conventional charge pump uses the close loop schema and the load changes, the charge pump is not able to adjust the output voltage instantly with precision.
Secondly, if the conventional charge pump uses the close loop schema and adopts a comparator for comparing the output voltage and a reference voltage to generate a feedback control signal, the feedback control signal is used to generate at least one clock signal for switching the charge pump. Such a comparator type schema results in an unstable system.